[PATCH] D44367: [InstCombine] peek through FP casts for sign-bit compares (PR36682)

[Sanjay Patel via Phabricator via llvm-commits]

spatel created this revision.
spatel added reviewers: majnemer, efriedma, arsenm.
Herald added subscribers: wdng, mcrosier.

This pattern came up in PR36682:
https://bugs.llvm.org/show_bug.cgi?id=36682
https://godbolt.org/g/LhuD9A

The 'undef' in the vector test is just there to verify a recent improvement in pattern-matching: vector constant matches can include undef elements.


https://reviews.llvm.org/D44367

Files:
  lib/Transforms/InstCombine/InstCombineCompares.cpp
  test/Transforms/InstCombine/icmp.ll


Index: test/Transforms/InstCombine/icmp.ll
===================================================================
--- test/Transforms/InstCombine/icmp.ll
+++ test/Transforms/InstCombine/icmp.ll
@@ -3315,9 +3315,7 @@
 
 define i1 @doublecast_signbit_set(i64 %x) {
 ; CHECK-LABEL: @doublecast_signbit_set(
-; CHECK-NEXT:    [[F:%.*]] = sitofp i64 [[X:%.*]] to float
-; CHECK-NEXT:    [[I:%.*]] = bitcast float [[F]] to i32
-; CHECK-NEXT:    [[R:%.*]] = icmp slt i32 [[I]], 0
+; CHECK-NEXT:    [[R:%.*]] = icmp slt i64 [[X:%.*]], 0
 ; CHECK-NEXT:    ret i1 [[R]]
 ;
   %f = sitofp i64 %x to float
@@ -3328,9 +3326,7 @@
 
 define <3 x i1> @doublecast_signbit_clear(<3 x i32> %x) {
 ; CHECK-LABEL: @doublecast_signbit_clear(
-; CHECK-NEXT:    [[F:%.*]] = sitofp <3 x i32> [[X:%.*]] to <3 x double>
-; CHECK-NEXT:    [[I:%.*]] = bitcast <3 x double> [[F]] to <3 x i64>
-; CHECK-NEXT:    [[R:%.*]] = icmp sgt <3 x i64> [[I]], <i64 -1, i64 undef, i64 -1>
+; CHECK-NEXT:    [[R:%.*]] = icmp sgt <3 x i32> [[X:%.*]], <i32 -1, i32 -1, i32 -1>
 ; CHECK-NEXT:    ret <3 x i1> [[R]]
 ;
   %f = sitofp <3 x i32> %x to <3 x double>
Index: lib/Transforms/InstCombine/InstCombineCompares.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -4506,6 +4506,17 @@
         return New;
   }
 
+  // Sign-bit checks are preserved through signed floating-point casts:
+  // icmp slt (bitcast (sitofp X)),  0 --> icmp slt X,  0
+  // icmp sgt (bitcast (sitofp X)), -1 --> icmp sgt X, -1
+  Value *X;
+  if (match(Op0, m_BitCast(m_SIToFP(m_Value(X))))) {
+    if (Pred == ICmpInst::ICMP_SLT && match(Op1, m_Zero()))
+      return new ICmpInst(Pred, X, ConstantInt::getNullValue(X->getType()));
+    if (Pred == ICmpInst::ICMP_SGT && match(Op1, m_AllOnes()))
+      return new ICmpInst(Pred, X, ConstantInt::getAllOnesValue(X->getType()));
+  }
+
   // Test to see if the operands of the icmp are casted versions of other
   // values.  If the ptr->ptr cast can be stripped off both arguments, we do so
   // now.


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